You must complete 175 hours every 5 years, following the issuance of your Professional Certificate.
The Professional Development year is July 1st-June 30th. You must start accruing hours beginning on the July 1st after your certificate has been issued. PD accrued prior to July 1st of that year will not count.
If you have more than one Professional Certificate, you only need to complete 175 hours within 5 years, not 175 x 2. However, if your certificates were not issued at the same time, you need to submit for each at the end of the 5 year period--though the PD hours during the overlap can count for both.
Even if you are not working in the area for which you hold a Professional Certificate, you must complete the hours in the 5 years time if you want to maintain that certificate.
You do not have to complete 35 hours per year, totalling 175 hours at the end of five years. You could, conceivably, complete all of them in one year if you wanted to. For obvious reasons, however, that is not suggested.
Here is some information about the 175 hours: